DRAINAGE CALCULATIONS

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Marilyn Black
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1 DRAINAGE CALCULATIONS For the Carroll Single Family Residence At Santa Clara County Saratoga, California APN: December 18, 2015 Prepared For: John and Peggy Carroll Prepared By: RI Engineering, Inc. Project Number /18/2015

2 Carroll Single Family Residence Design Criteria Storm drainage calculations described in this document have been done in conformance with the Santa Clara County Drainage Manual Project Description The project consists of constructing a single family residence with driveway, garage, concrete patio and a pool on a hilltop lot in the Saratoga mountain area of Santa Clara County. The project will create approximately 7,205 square feet of impervious area. Existing Conditions The lot is currently undeveloped. There is no impervious area on the site. The Mean Annual Precipitation (M.A.P) for the area is 28 per Santa Clara County Drainage Manual Figure A-2. The soil type in the improvement area is Katykat-Sanikara complex for 8 to 30% slopes and Mouser- Katykat-Sanikara complex for 50 to 75% slopes. An infiltration rate for the existing soils has been determined to be 1.6 inches per hour (from USDA Web Soil Survey). The C factor for the site is 0.35 per Santa Clara Drainage Manual Table 3-1. Runoff sheet flows over the site toward the eastern property line. Based on the Rational Method, the peak predevelopment runoff rate for the site is 0.18 cubic feet per second (cfs) for a 10-year storm event and 0.26 cfs for a 100-year storm event (Table 1). Proposed Development The proposed improvements for the subject project are to construct an single family residence with a driveway, rear patio, and pool. Runoff from the roof downspouts will be captured in closed conduit stormdrains per the geotechnical report. Runoff from all other impervious surfaces will be captured in catch basins and stormdrains. The stormdrains will discharge to gravel filled level spreaders at the downslope edge of the site. The level spreaders will store water and serve as infiltration trenches before overtopping and discharging runoff to sheet flow downslope per existing conditions. The post-development runoff rate for a 10-year design storm event is 0.32 cfs and for a 100-year design storm event is 0.46 cfs (Table 1). The infiltration level spreaders have been sized to store and infiltrate the increase in stormwater. Peak runoff leaving the site has been maintained per predevelopment rates for the 10 and 100 year design storm (see table 2). 1

3 Carroll Single Family Residence Conclusion The proposed project is to construct a new single family residence that will create 7,205 square feet of impervious area. Runoff from the new impervious area will be collected in downspouts and drop inlets and conveyed to infiltration level spreaders which release the runoff to match pre-development rates. No adverse downstream effects are anticipated as part of this project, and the drainage patterns leaving the site have not be changed. Attachments Figure 1: M.A.P Table 1: Peak Runoff Calculations Table 2: Retention Storage Calculations 2

com/permit_c3_docs/ C3_Handbook/Handbook_

4 Drainage Manual 2007 County of Santa Clara, California Location of Map: C3_Handbook/Handbook_ May_2006-Oct_update.pdf Figure A 2: Mean Annual Precipitation, Santa Clara County 8/14/2007 A 4

5 Carroll Residence Saratoga, CA HYDROLOGY Drainage Basin: 10-year and 100-year storm event Total Basin Area 13,000 ± SF 0.30 ± AC Determine PRE Development Runoff Coefficient: C Feature Area Area C AxC (sf) (acres) Pervious 13, Impervious Totals: 13, Pre Development C AVERAGE = 0.35 Time of Concentration: Pre- Tc = 15 mins Determine Pre Developed Q for a 10-year and 100-year storm. 10-year 100-year Pre- I (in/hr)*= Q Pre Developed (cfs)= Determine POST Development Runoff Coefficient: C Feature Area Area C AxC (sf) (acres) Pervious 5, Impervious 7, Total 13, Post Development C AVERAGE = 0.63 Time of Concentration: Post- Tc = 15 mins Determine Post Developed Q for 10-year and 100-year storm. 10-year 100-year Post- I (in/hr)*= Q Post Developed (cfs)= Q (cfs)= * from Santa Clara County 2007 Drainage Manual (Fig. B-5: IDF for 28" M.A.P.) Table 1

6 Carroll Residence Saratoga, CA DESIGN STORM RETENTION CALCULATIONS Storage Volume for Retention Calculation Area= 13,000 sf Cpre= 0.35 Cpost= 0.63 Qpre10= cfs (10-year) Qpre100= cfs (100-year) Ksat= 1.6 in/hr Void Ratio 0.4 Infiltration Level Spreader Combined Length (ft)= 60 Width (ft)= 4 Depth of Storage (ft)= 2 Perimeter (ft)= 128 Surface Area (sf)= 496 Effective Area Factor= 75% TOTALS Effective Surface Area (sf)= Footprint Area (sf)= Excavated Volume (cf)= Storage Volume (cf)= Structure Infilration Rate (cfs)= Structure Drainage Time (hr)= Year Storm - Storage Volume Calculation Storm Duration Rainfall Intensity Qpost Infiltration Rate Soil Infiltration Rate Storage Volume min in/hr cfs cfs cfs cf Storage Volume= Provided Storage Volume= 128 cf 192 cf 100- Year Storm - Storage Volume Calculation Storm Duration Rainfall Intensity Qpost Infiltration Rate Soil Infiltration Rate Storage Volume min in/hr cfs cfs cfs cf Storage Volume= Provided Storage Volume= 191 cf 192 cf Table 2

LID Permit Requirements. Lisa Austin. Geosyntec Consultants

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